This invention relates generally to cleaning apparatus and, more particularly, to apparatus in which parts in a bath of liquid are cleaned by virtue of vibrational wave energy causing the liquid to impinge on the parts to loosen sedimentation and surface contamination. The frequency of the vibrational wave energy usually is in the ulrasonic range although in some few cases the frequency may be in the lower sonic or audible range. Simply for purposes of brevity and convenience, the invention shall hereafter be described in this specification in conjunction with an ultrasonic system but, in the appended claims, the term "acoustic" should be considered to encompass both sonic and ultrasonic systems.
In conventional ultrasonic cleaning apparatus, the liquid bath for the parts is contained in a tank or the like. One or more electric-to-ultrasonic vibration transducers are mounted on a flat vibration-transmitting base or plate fixed to the lower side of the bottom wall of the tank. When the transducer means are electrically excited by an ultrasonic frequency oscillator, vibrational waves are produced and travel upwardly from the bottom of the tank to the top surface of the liquid. Upon reaching the top surface, the energy waves are, to some extent, reflected back into the bath.
If the transducer means produce waves of an essentially single and constant frequency, and assuming the transducer means are located at the bottom of the tank, a substantially uniform pattern of standing waves is set up in the liquid. As the depth of the liquid in the tank changes, wave reflection from the upper surface of the liquid will change in intensity but the pattern of the standing waves in the liquid will remain substantially uniform, that is, all standing waves will have essentially the same vertical locations of peaks and nulls. As a result, the peaks and nulls of the wave pattern occur at certain levels in the liquid and remain at those levels during the entire cleaning cycle, the peak amplitudes of the standing waves remaining essentially constant as long as the depth of the liquid is constant. This produces a non-uniform cleaning action along the height of a part disposed in the liquid. Moreover, the uniformity of efficiency of the cleaning action changes as the liquid level changes because the intensity of the energy refelcted back into the bath varies as the liquid depth changes over the span of a quarter wave length. Changes in liquid level changes the mechanical (and thus the overall) resonant frequency of the system and this results in significant variations in the output power of the ultrasonic generator and input power to the transducer means even though no change takes place in the output voltage of the frequency generator.
The problems created by a standing wave pattern at an essentially single chosen frequency have been recognized in the art of ultrasonic cleaning. Various solutions have been proposed. In this regard, reference is made to Tomes U.S. Pat. No. 3,254,284; Cook U.S. Pat. No. 3,371,233; Kennedy et al U.S. Pat. No. 4,120,699 and Ratcliff U.S. Pat. No. 4,554,477. Except for Kennedy et al, these solutions involve either exciting the bath with multile frequencies or sweeping the excitation frequency over a range as time passes, so that the bath contains vibrational waves of different wave lengths. In the case of Kennedy et al, a plurality of transducers are spaced around a tank in somewhat opposed configuration to one another so that opposed vibratory radiations create an interference pattern which breaks up the pronounced effect of single frequency standing waves. Multiple frequency or sweep frequency generators and transducers are significantly more complex and expensive than single frequency apparatus; and opposed transducers are likewise an expensive approach to the problem.